GENETIC CONTROLGENETIC CONTROL

The structure and replication of The structure and replication of DNADNA

ALBIO9700/2006JK

Structure of DNA and RNAStructure of DNA and RNA

• DNA – deoxyribonucleic acidDNA – deoxyribonucleic acid• RNA – ribonucleic acidRNA – ribonucleic acid• Polynucleotides/nucleic acidsPolynucleotides/nucleic acids• NucleotidesNucleotides

– Made up of 3 smaller components: nitrogen-Made up of 3 smaller components: nitrogen-containing base; pentose sugar; phosphate group)containing base; pentose sugar; phosphate group)

– 5 bases: adenine, thymine/uracil, guanine, 5 bases: adenine, thymine/uracil, guanine, cytosinecytosine

– Pentose (5-carbon): ribose (RNA) and deoxyribose Pentose (5-carbon): ribose (RNA) and deoxyribose (DNA)(DNA)

ALBIO9700/2006JK

ALBIO9700/2006JK

• PolynucleotidesPolynucleotides– Many nucleotides linked in a long chain (during Many nucleotides linked in a long chain (during

interphase of cell cycle)interphase of cell cycle)– Formed of alternating sugars and phosphates linked Formed of alternating sugars and phosphates linked

together, with the bases projecting sidewaystogether, with the bases projecting sideways– DNA molecule: 2 polynucleotide strands lying side by DNA molecule: 2 polynucleotide strands lying side by

side running in opposite directions (held together by side running in opposite directions (held together by hydrogen bondshydrogen bonds between bases) between bases)

– Bases can be Bases can be purinespurines (adenine and guanine) or (adenine and guanine) or pyrimidines pyrimidines (cytosine and thymine)(cytosine and thymine)

– Complementary base pairingComplementary base pairing (A with T, C with G) (A with T, C with G)– Double helix: the 3D shape that DNA molecules formDouble helix: the 3D shape that DNA molecules form– RNA molecules remain as single strands of RNA molecules remain as single strands of

polynucleotide and form a different 3D structurepolynucleotide and form a different 3D structure

ALBIO9700/2006JK

ALBIO9700/2006JK

ALBIO9700/2006JK

DNA replicationDNA replication

• Ability to be copied perfectly many Ability to be copied perfectly many times overtimes over

• 1953: James Watson and Francis 1953: James Watson and Francis Crick (Rosalind Franklin)Crick (Rosalind Franklin)

• Semi-conservative replicationSemi-conservative replication: : half of the original molecule is kept half of the original molecule is kept (conserved) in each of the molecules(conserved) in each of the molecules

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1.1. The DNA double helix unwinds and ‘unzips’ The DNA double helix unwinds and ‘unzips’ as the hydrogen bonds between the bases as the hydrogen bonds between the bases breakbreak

2.2. In the nucleus, there are nucleotides to In the nucleus, there are nucleotides to which 2 extra phosphates have been which 2 extra phosphates have been added. The extra phosphates activate the added. The extra phosphates activate the nucleotides, enabling them to take part in nucleotides, enabling them to take part in the following reactionsthe following reactions

3.3. The bases of the activated nucleotides pair The bases of the activated nucleotides pair up with their complementary base on each up with their complementary base on each of the old DNA strands. An enzyme, DNA of the old DNA strands. An enzyme, DNA polymerase, links the sugar and innermost polymerase, links the sugar and innermost phosphate groups of next-door nucleotides phosphate groups of next-door nucleotides together. The two extra phosphates are together. The two extra phosphates are broken off and released into the nucleus.broken off and released into the nucleus.

• DNA polymerase will only link an incoming DNA polymerase will only link an incoming nucleotide to the growing new chain if it is nucleotide to the growing new chain if it is complementary to the base on the old strand. complementary to the base on the old strand. Thus very few mistakes are made, perhaps Thus very few mistakes are made, perhaps around one in every 10around one in every 1088 base pairs. base pairs.

ALBIO9700/2006JK

ALBIO9700/2006JK